export const getBase64 = function (imgUrl) {
    return new Promise((resolve,reject)=>{
    window.URL = window.URL || window.webkitURL;
    var xhr = new XMLHttpRequest();
    xhr.open("get", imgUrl, true);
    // 至关重要
    xhr.responseType = "blob";
    xhr.onload = function () {
        if (this.status == 200) {
            //得到一个blob对象
            var blob = this.response;
            console.log("blob", blob)
            // 至关重要
            let oFileReader = new FileReader();
            oFileReader.onloadend = function (e) {
                let base64 = e.target.result;
                //使用压缩
                dealImage(base64, 200, (newBase64) => {
                    console.log("压缩前", base64.length / 1024)
                    console.log("压缩后", newBase64.length / 1024)
                    resolve(newBase64)
                    //====为了在页面显示图片，可以删除====
                    // var img = document.createElement("img");
                    // img.onload = function (e) {
                    //     window.URL.revokeObjectURL(img.src); // 清除释放
                    // };
                    // let src = newBase64;
                    // img.src = src
                    // document.getElementById("container1").appendChild(img);
                    //====为了在页面显示图片，可以删除====
                });
            };
            oFileReader.readAsDataURL(blob);
        }
    }
    xhr.send();
})
}

function dealImage(base64, w, callback) {
    var newImage = new Image();
    var quality = 0.6;    //压缩系数0-1之间
    newImage.src = base64;
    newImage.setAttribute("crossOrigin", 'Anonymous');	//url为外域时需要
    var imgWidth, imgHeight;
    newImage.onload = function () {
        imgWidth = this.width;
        imgHeight = this.height;
        var canvas = document.createElement("canvas");
        var ctx = canvas.getContext("2d");
        if (Math.max(imgWidth, imgHeight) > w) {
            if (imgWidth > imgHeight) {
                canvas.width = w;
                canvas.height = w * imgHeight / imgWidth;
            } else {
                canvas.height = w;
                canvas.width = w * imgWidth / imgHeight;
            }
        } else {
            canvas.width = imgWidth;
            canvas.height = imgHeight;
            quality = 0.6;
        }
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
        var base64 = canvas.toDataURL("image/jpeg", quality); //压缩语句
        // 如想确保图片压缩到自己想要的尺寸,如要求在50-150kb之间，请加以下语句，quality初始值根据情况自定
        // while (base64.length / 1024 > 150) {
        // 	quality -= 0.01;
        // 	base64 = canvas.toDataURL("image/jpeg", quality);
        // }
        // 防止最后一次压缩低于最低尺寸，只要quality递减合理，无需考虑
        // while (base64.length / 1024 < 50) {
        // 	quality += 0.001;
        // 	base64 = canvas.toDataURL("image/jpeg", quality);
        // }
        callback(base64);//必须通过回调函数返回，否则无法及时拿到该值
    }
}